N = R* x fP x nE x fL x fl x fC x L
- N = the number of technically advanced civilizations active in the Milky Way capable of broadcasting radio transmissions
- R* = the rate at which stars suitable for hosting habitable planets are formed in the galaxy each year
- fP = the fraction of those stars that have planets
- nE = the number of Earth-like planets per solar system capable of incubating and supporting life
- fL = the fraction of Earth-like planets will life will develop
- fl = the fraction of planets with life that develop intelligence
- fC = the fraction of planets with intelligent life that develop the technology to communicate
- L = the lifetime of a technical civilation
Astronomer Frank Drake devised this formula of probability in 1961. The last four components have values that are considered to be highly speculative. Another astronomer with an ardent interest in extraterrestrial life, Carl Sagan, estimated that the value of N must equal at least one million civilizations in the Milky Way. This does not include other parts of the universe.
